1. Field of the Invention
This invention relates to means and methods for determining magnetic and electric fields using gradiometers, magnetometers, or electrometers. More particularly, perturbations in magnetic and electric fields due to magnetic or electric dipoles are sensed with electric or magnetic gradiometers.
2. Prior Art
Many devices for measuring magnetic field direction, locating objects or determining the orientation of an object by measuring electromagnetic fields have been proposed. Among these are U.S. Pat. Nos. 3,061,239 to S. J. Rusk, 3,644,825 to P.D. Davis, Jr. et al, 3,728,525 to C. K. Adkar, 4,267,640 to C. T. Wu, 4,287,809 to W. H. Egli, et al, 4,302,746 to J. F. Scarzello, et al and 4,314,251 to F. H. Raab.
Raab uses generalized matrix formulations as the basis for a system of radiating and receiving antennas which determine the position and orientation of a remote object. The transmitter employs two orthogonal radiating antennas and the receiver has three mutually orthogonal receiving antennas. The transmitting antennas should be magnetic dipole sources. Raab discloses only the intentional excitation of transmission loops by periodic signals. His equations for determining position and orientation are of the most general form (e.g. equation 27 is a general product matrix for three orthogonal rotations) and are quite complex.
Similarly the Egli, et al patent determines orientation and position of a helmet with a system having transmitting and receiving antennas by employing generalized matrix formulations.
Rusk discloses a static magnetic moment device for maintaining a satellite in a predetermined orientation with respect to its orbit about the earth. Rusk makes use of the torque produced by the magnetic interaction between the earth's magnetic field and a predetermined magnetic field developed on the satellite in response to attitude control signals derived from conventional vehicle attitude detection devices. Three mutually perpendicular magnetic torquing coils are utilized.
Wu measures a magnetic field with crossed rods each having a rectangular shaped B-H hysteresis curve. The earth's field biases each rod so that the B field of each will switch between a high and low state with voltages in direct proportion to the component of the earth's field line along each rod.
Scarzello, et al uses a two axis magnetometer to sense a vehicle's magnetic signature. Standard two axis magnetometers or gradiometers with windings on ring cores are integrated in a system to sense the arrival and exit of a vehicle at a fixed location. Comparisons to predetermined thresholds are made to screen against electromagnetic interference effects and false alarms.
Adkar determines the geographical location of an object on the earth by imposing known perturbations of magnetic flux first on the vertical component of the earth's field and then on an orthogonal component lying in a horizontal plane at the earth's surface. Knowledge of total field strength allows the determination of location, inclination and azimuth.
Davis, Jr., et al uses two magnetic field sensors to generate output signals representative of perpendicular directional components of a varying magnetic field. Each output signal is differentiated and circuitry multiplies each output signal by the differential of the other output signal. The multiplication products are substracted to produce a resultant signal. The polarity and magnitude of the resultant signal is sensed to determine either direction of movement of the object creating the magnetic field, or to indicate the relative position of the object with respect to the sensors if the direction of movement of the object is known.
Also known are the field equations for magnetic induction B(r) due to a magnetic dipole, i.e.: ##EQU1## where r is the direction vector between an origin on the dipole and the point of observation, u is a unit vector from the origin in the direction of r and m is the magnetic moment defined by: ##EQU2## for a current distribution J (see FIG. 1). For a ferromagnetic material m is the sum of a permanent magnetic moment and an induced magnetic moment.
Similar equations hold for the electric field E(r) due to a dipole of electric dipole moment p, i.e. ##EQU3## where .rho. is a charge distribution.
None of the above systems provide for the determination in a plane (from two orthogonal magnetic field components due to the perturbation of an external magnetic field by the magnetic dipole) of the angular orientation of a magnetic dipole relative to a point of observation with a single equation having one independent variable. Nor do such prior art systems provide for the same determination from perturbations of an electric field due to an electric dipole.